1. Field of the Invention
The present invention relates to a method of recovering energy from bituminous sedimentary rock, in particular oil shales, in which retorted shale is crushed and then finely divided, to a particle size of sufficient fineness to enable the major portion of the coke and any minerals present to be separated out.
2. Description of the Prior Art
In the prior art, raw shale is crushed and placed into a retort under retorting conditions to obtain hydrocarbon products. After sufficient hydrocarbon products have been obtained, retorted shale may generally be processed to recover heat or synthesis gas. Depending upon the process additional processing of the retorted shale may be necessary since it retains mineral matter and a high energy coke residue. Temperatures required to obtain heat and synthesis gas from the retorted shale are quite high, generally from about 600.degree.-1000.degree. C. At these elevated temperatures mineral decomposition reactions proceed at a rapid speed. Mineral decomposition reactions are endothermic and produce large volumes of carbon dioxide plus other pollutants. Also, thermal efficiency obtained during the retorting stage is lost in the processing of the retorted coke by conventional means.
Those skilled in the prior art have known that surface retorts can be used to recover oil products from oil shale. Allred, U.S. Pat. No. 3,960,702 is an example of the use of a surface retort to recover oil products from oil shale and this patent is hereby incorporated by reference. Brown, U.S. Pat. No. 4,094,769, also used a surface retort to recover oil products from shale. This patent is also hereby incorporated by reference.
In order to retort crushed raw oil shale, it is necessary to use elevated temperatures. Temperatures used in retorting generally are at least as high as 450.degree. C. It is recognized that the residence time should be as short as possible to prevent the coke formation by hydrocarbon vapors released from the retorted oil shale. Since coke formation was not desired, others have sought ways to minimize coke formation and still maintain a high level of hydrocarbon recovery. Coke formation was not desired because to convert it into another form of energy, it was necessary to heat the retorted shale which comprised the inorganic matrix as well as the coke. Heating the coke to a temperature sufficient to transform it to another energy source required even higher temperatures than retorting the raw shale. These temperatures were generally in the range of from about 500.degree. to 1000.degree. C. At these elevated temperatures mineral decomposition of the inorganic matrix occurred which causes the formation of undesired gases which may be classified as environmental pollutants.
In order to avoid the use of high temperatures with attendant mineral decomposition and coking problems, Fahlstrom, U.S. Pat. No. 4,176,042 sought to crush retorted shale into finely divided particles in comminuting stages and subject the particles to physical and chemical separation processes in order to recover the desired oil and inorganic products. This recovery was accomplished by separating the kerogen contained in the raw shale from the inorganic matrix of the rock. After subjecting the raw shale to an initial crushing and comminution stage, separation processes were used. Sequential crushing, comminuting, and separating stages were continued until the desired kerogen inorganic components were obtained.
Until now perhaps an efficient method did not exist for recovering heat values from retorted shale. Allred and Brown taught exposing the uncrushed retorted shale to conventional methods to recover heat values or synthesis gas from the coke entrained on the oil shale. This was done either by the combustion method or by injecting superheated water vapor into a reactor to recover synthesis gas from the retorted shale. Allred taught retorting the shale and disposing of it after cooling. Brown, on the other hand, taught sequentially crushing and then retorting raw shale. No provision was made by Brown for utilizing the retorted shale to recover heat gained during processing. Therefore, neither Allred or Fahlstrom taught a method for the further processing of retorted shale to recover heat values.
In accordance with the present invention it will not be necessary to lower the temperature when retorting oil shale. Thus, hydrocarbon production can be maximized and coking need not be delayed. Increased coking will provide for additional energy which can be used for maintaining the retort temperature. Energy can also be conserved by feeding a larger size of crushed oil shale. Because larger sizes of oil shale can be used in the retort, crushing times for raw oil shale can be reduced. After separating the coke from the inorganic matrix, less heat will be required to convert the coke into other energy forms. Additionally, air pollutants should be minimized since the inorganic matrix will not be subjected to elevated temperatures which led to mineral decomposition.